mount: Add IO limit group classifier

Add a function which determines IO limiting group of a local process.
mount: Introduce I/O limiting mechanism

This change makes it possible to define I/O limits for processes
from different cgroups in one hierarchy (identified by a subsystem,
eg. blkio). For example this configuration:

  subsystem blkio
  limit /group1 1024
  limit /group2 30720

limits I/O generated by processes from /group1 to 1 MB/s and by processes
from /group2 to 30 MB/s. This throughput is defined as a sum of read and
write requests (on the interface between then kernel and mount).

To enable I/O limiting, mount the filesystem with mfsiolimits=<file> option
providing a path to a file with configuration of these limits.
tests: Add a simple test of I/O limits